There are different alternatives for treating sludge. For example, there is known digestion that is anaerobic treatment, as well as composting that is aerobic treatment, i.e. treatment utilizing oxygen. Further, thermal treatment is known, in other words treatment taking place by heating.
Of treatments taking place by heating, for instance drying of sludge with direct hot air is known, but a disadvantage of this technique is the large amount of energy required and dust formation in the material, resulting from the drying. Of methods utilizing heating, also sludge treatment with hot water is known, but a problem with this method is that the water used in the method must be purified, which causes costs. Further, of treatments taking place by heating, methods using ordinary water vapour are known but according to the applicant's observations, they are not completely satisfactory as regards the efficiency of purification and thermal economy.
In purifying waste or other material, the use of superheated steam is also known. Superheated steam is a gas mixture formed of water vapour and combustion gas. Some known solutions are presented in publications U.S. Pat. No. 5,656,178, U.S. Pat. No. 5,613,452, U.S. Pat. No. 5,413,129, DE 4226584, EP 0715902, U.S. Pat. No. 4,336,329, WO 01/02027 and WO 02/28556, but the publications relate to sterilization of material, which also excessively eliminates the useful microorganisms, preventing thus utilization of the material as fertilizer.
Relating to the field of the present invention, a method using superheated steam is known from publication EP 101784 for manufacturing fertilizer out of bark and sludge. In the method disclosed in this publication, sludge is heated to a temperature of 120 to 140° C. by using superheated steam having a temperature of 140 to 600° C. However, the method does not give optimal results with regard to the fertilizer properties of the final product and the thermal economy of the method because of, in particular, the high temperature to which the material to be treated is heated.